Swashplate-type axial piston pumps are commonly used devices used for generating hydraulic power. These pumps may be configured to provide a variable displacement flow for greater energy efficiency when compared with a non-variable displacement hydraulic pump. The pump component that provides variable displacement flow is a movable, inclined cam plate, typically referred to as a swashplate. The swashplate has a running surface upon which a group of pistons slide or travel. The swashplate ordinarily pivots on either roller or plain bearings. In either bearing case, the bearings are generally designed to provide low friction and long service life. Low friction bearings ensure proper controllability of the outlet flow. The benefits of long service life include lower lifecycle cost and reduced downtime. Other considerations in the design and selection of the swashplate bearings include size, weight, and initial cost among others. The benefits of reduced size, weight, and initial cost may be achieved by using plain swashplate bearings in lieu of roller bearings. The plain bearings are typically referred to as cradle bearings due to their semi-cylindrical shape. For an example of such cradle bearings, refer to U.S. Pat. No. 4,543,876 incorporated herein by reference in its entirety.
To reduce bearing friction to acceptable levels and to minimize wear, some plain bearing designs incorporate hydrostatic pockets in a semi-circular swashplate backside surface. The hydrostatic pockets may be supplied with pressurized fluid from a discharge port of the pump typically via transfer tubes or passages in the pump's housing.
Swashplate pumps are ordinarily designed such that the running surface of the swashplate is inclined solely about a single (primary) axis, which is perpendicular to the pump's driveshaft. The variable inclination of the swashplate about this axis allows for variable output flow. Some swashplate pumps also incorporate a second swashplate angle that is fixed at typically less than 4 degrees with respect to the variable plane passing through the primary axis. The benefits of incorporating this secondary swashplate angle may include a reduction in the size of the swashplate control mechanism (allowed by reducing the torque exerted on the swashplate by the pumping pistons). The smaller swashplate control mechanism may reduce the corresponding pump's size and weight.
Additionally, the pump's timing may be adjusted by the secondary swashplate angle to reduce fluid-borne noise.
The disadvantages of incorporating a secondary swashplate angle may include higher manufacturing costs and the need to react the side force exerted by the secondary angle. The thrust load developed by this side force can be approximately 5% of the thrust load reacted by the swashplate bearings. A common method of reacting this side force is via non-pressurized thrust bearings. These bearings may increase manufacturing costs, increase friction that impedes swashplate motion, and generate damaging wear particles due to the nearly continuous motion at the bearing interface.